P. Beiss - Institute of Materials Applications in Mechanical Engineering, RWTH Aachen University, Germany

Estimates of fully reversed fatigue strengths of ductile pore-free steels are usually based on tensile strengths. For porous sintered steels this approach has limited value because at carbon contents exceeding 0.6 % tensile strengths start to decrease, whereas hardness and fully reversed fatigue strengths continue to increase. For this reason a linear hardness scale, like Vickers or Brinell, should be better suited for estimates, if a wide range needs to be covered. To develop a mathematical relationship between hardness and fully reserved bending fatigue strength, more than 100 S-N curves with documented hardness data were drawn from the literature, ranging from pure as-sintered iron to heat treated alloy steels. Applying Balshin’s equation to the density dependencies of hardness and fatigue strength, average density exponents were determined. With these exponents the ratio of hardness and fatigue strength can be expressed using the pore-free state as a reference. This procedure delivered an estimate of about ± 15 % around the measured fatigue strength. A refinement to about ± 10 % was achieved if the references was changed to the density of 7.0 g/cm3.